Spacer Etching Process for Integrated Circuit Design

ABSTRACT

A method includes forming a first material layer on a substrate and performing a first patterning process using a first layout to form a first plurality of trenches in the first material layer. The method further includes performing a second patterning process using a second layout to form a second plurality of trenches in the first material layer, wherein the second layout a cut pattern for the first layout. The method further includes forming spacer features on sidewalls of both the first and second pluralities of trenches, wherein the spacer features have a thickness and the cut pattern corresponds to a first trench of the second plurality whose width is less than twice the thickness of the spacer features. The method further includes removing the first material layer; forming a second material layer on the substrate and within openings defined by the spacer features; and removing the spacer features.

This is a divisional application of U.S. patent application Ser. No.14/081,345, entitled “Spacer Etching Process for Integrated CircuitDesign,” filed Nov. 15, 2013, which claims the benefit of U.S.Provisional Application No. 61/791,138 entitled “Spacer Etching Processfor Integrated Circuit Design” filed Mar. 15, 2013. The entirety of bothapplications is hereby incorporated by reference. This patentapplication also hereby incorporates by reference U.S. patentapplication Ser. No. 13/892,945 entitled “A Method of Fabricating aFinFET Device” filed May 13, 2013.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experiencedexponential growth. Technological advances in IC materials and designhave produced generations of ICs where each generation has smaller andmore complex circuits than the previous generation. In the course of ICevolution, functional density (i.e., the number of interconnecteddevices per chip area) has generally increased while geometry size(i.e., the smallest component (or line) that can be created using afabrication process) has decreased. This scaling down process generallyprovides benefits by increasing production efficiency and loweringassociated costs. Such scaling down has also increased the complexity ofprocessing and manufacturing ICs and, for these advances to be realized,similar developments in IC processing and manufacturing are needed.

For example, a spacer technique is often used to form mandrels, whichare used in devices such as a fin field effect transistor (FinFET)device. Frequently, the spacer technique is used for doubling theexposed pattern in advanced lithography. That is, the pitch of a finalpattern is reduced to only half compared with the first exposed pattern.Due to constraints from the lithography process, it is difficult toobtain small cut features.

Also in some occasions, it is desirable to have a large process window.The process window refers to a range of focus and exposure settings thatwill still produce the desired features into the photo-resist layer inthe photolithographic process.

Accordingly, what is needed is an improvement in this area.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIG. 1 is a flow chart of a method of forming a target pattern on asubstrate for implementing one or more embodiments.

FIG. 2 shows a target pattern 200 with target features 202, 204, 206,208, 210, and 212. FIG. 2 also shows the target feature 208 being cutfrom the target feature 206 by a cut feature 214.

FIGS. 3 a, 3 b, 3 c, 4 a, 4 b, 5 a, and 5 b illustrate the operations offorming the target pattern 200 according to the method of FIG. 1, inaccordance with an embodiment.

FIGS. 6 a, 6 b, 6 c, 6 d, 6 e, 7 a, 7 b, 7 c, 8 a, 8 b, 8 c, 9 a, 9 b, 9c, 9 d, 9 e, 10 a, 10 b, 10 c, 10 d, 10 e, 11 a, 11 b, 11 c, 11 d, 11 e,11 f, and 11 g are top and cross sectional views of forming a deviceaccording to the method of FIG. 1, in accordance with an embodiment.

FIGS. 12 a and 12 b illustrate the minimum cut feature with twoembodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the disclosure.Specific examples of components and arrangements are described below tosimplify the present disclosure. These are, of course, merely examplesand are not intended to be limiting. In addition, the present disclosuremay repeat reference numerals and/or letters in the various examples.This repetition is for the purpose of simplicity and clarity and doesnot in itself dictate a relationship between the various embodimentsand/or configurations discussed. Moreover, the performance of a firstprocess before a second process in the description that follows mayinclude embodiments in which the second process is performed immediatelyafter the first process, and may also include embodiments in whichadditional processes may be performed between the first and secondprocesses. Various features may be arbitrarily drawn in different scalesfor the sake of simplicity and clarity. Furthermore, the formation of afirst feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. For example, if the device in the figures is turned over,elements described as being “below” or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.Thus, the exemplary term “below” can encompass both an orientation ofabove and below. The apparatus may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein may likewise be interpreted accordingly.

Referring now to FIG. 1, a flow chart of a method 100 for forming atarget pattern is illustrated. Additional operations can be providedbefore, during, and after the method 100, and some operations describedcan be replaced, eliminated, or moved around for additional embodimentsof the method. The method 100 is an example, and is not intended tolimit the present disclosure beyond what is explicitly recited in theclaims. The method 100 will be further described below.

FIG. 2 is a diagram showing an example target pattern 200 including anumber of target features 202, 204, 206, 208, 210, 212 and a cut feature214. The target features may be, for example, metal lines. This targetpattern 200 will be further described with reference to the additionalfigures of the patent, as discussed below.

Referring to FIGS. 1 and 3 a, the method 100 begins at operation 102 byproviding a substrate 218. The substrate 218 includes one or morematerial layers. In an embodiment, the substrate includes asemiconductor layer, a pad oxide layer, and a silicon nitride (SiN)layer. In an embodiment, the substrate includes a dielectric layer, aninter-layer dielectric layer such as an extreme low-k dielectric (ELK)layer, and an anti-reflection layer such as a nitrogen-freeanti-reflection coating (NFARC) layer. In one example, the NFARC layeruses a material such as silicon oxide, silicon oxygen carbide, or plasmaenhanced chemical vapor deposited silicon oxide.

The method 100 proceeds to operation 104 by forming a hard mask layer220 on the substrate 218. The hard mask layer 220 may include one ormore material layers and is formed by a procedure such as deposition. Inan embodiment, the hard mask layer 220 may include silicon oxide formedby thermal oxidation. In an embodiment, the hard mask layer 220 mayinclude SiN formed by chemical vapor deposition (CVD). For example, thehard mask layer 220 may be formed by CVD using chemicals includingHexachlorodisilane (HCD or Si2Cl6), Dichlorosilane (DCS or SiH2Cl2),Bis(TertiaryButylAmino) Silane (BTBAS or C8H22N2Si) and Disilane (DS orSi2H6).

Referring to FIGS. 1 and 3 b, the method 100 proceeds to operation 106by performing a first patterning process to the hard mask layer 220using a first layout, thereby forming a first plurality of trenches, 222a, 222 b, 222 c, and 222 d, in the hard mask layer 220, as illustratedin FIG. 3 b. The first patterning process includes a lithography processand an etching process. In an embodiment, a resist layer, patterned withthe first layout, is formed on the hard mask layer 220 using alithography process, such as resist coating, soft baking, exposing,post-exposure baking (PEB), developing, and hard baking in one example.Then, the hard mask layer 220 is etched through the openings of thepatterned resist layer, forming a plurality of trenches, 222 a, 222 b,222 c, and 222 d, in the hard mask layer 220 by the etching process. Thepatterned resist layer is removed thereafter using a suitable process,such as wet stripping or plasma ashing. In one example, the etchingprocess includes applying a dry (or plasma) etch to remove the hard masklayer 220 within the openings of the patterned resist layer.

Referring to FIGS. 1 and 3 c, the method 100 proceeds to operation 108by performing a second patterning process to the hard mask layer 220using a second layout, thereby forming a second plurality of trenches,224 a, 224 b, 224 c, and 224 d, in the hard mask layer 220, asillustrated in FIG. 3 c.

In an embodiment, the second patterning process starts with forming amaterial layer over the hard mask layer 220 using one or more materialdifferent from the hard mask layer 220. For example, while the hard masklayer 220 uses silicon oxide or silicon nitride, the material layer mayuse bottom anti-reflective coating (BARC) or spin-on glass (SOG). Thesecond patterning process further includes a lithography process and anetching process thereby forming the second plurality of trenches in thehard mask layer 220. In an embodiment, a resist layer, patterned withthe second layout, is formed on the material layer using a lithographyprocess. Then, the material layer and the hard mask layer 220 are etchedthrough the openings of the patterned resist layer, forming a pluralityof trenches in the hard mask layer 220 by the etching process. Thepatterned resist layer is removed thereafter using a suitable process,such as wet stripping or plasma ashing. The material layer is removedthereafter using a suitable process, such as an etching process tuned toselectively remove the material layer while the hard mask layer 220remains.

Thus far, by performing operations 106 and 108 of the method 100, boththe first plurality of mandrel trenches and the second plurality ofmandrel trenches are formed on the hard mask layer 220, and portions ofthe first plurality of mandrel trenches and portions of the secondplurality of mandrel trenches may merge.

FIG. 4 a shows the merged mandrel trenches in the hard mask layer 220including mandrel trenches 226 a, 222 b, 226 c, and 224 d. The mandreltrench 226 a is formed by merging mandrel trench 222 a formed inoperation 106 of the method 100 and mandrel trench 224 a formed inoperation 108 of the method 100. The mandrel trench 226 c is formed bymerging mandrel trenches 222 c and 222 d formed in operation 106 of themethod 100 and mandrel trenches 224 b and 224 c are formed in operation108 of the method 100.

Referring to FIGS. 1 and 4 b, after thus having formed the mergedmandrel trenches in the hard mask layer, the method 100 proceeds tooperation 110 by forming spacer features inside and on the sidewalls ofthe merged mandrel trenches, such as spacer features 228 a, 228 b, 228c, 228 d, and 228 e, as shown in FIG. 4 b. The spacer features have athickness. The spacer features include one or more materials differentfrom the hard mask layer 220, such as titanium nitride (TiN). Inaddition or in the alternative, the spacer features may include adielectric material, such as silicon oxide, silicon nitride, or siliconoxynitride. The spacer features can be formed by various processes,including a deposition process and an etching process. For example, thedeposition process includes a CVD process or a physical vapor deposition(PVD) process. For example, the etching process includes an anisotropicetch such as plasma etch. Wherein width of the mandrel trenches is equalto or less than twice the thickness of the spacer features, spacerfeatures merge inside the mandrel trenches. For example, referring toFIG. 4 b, within a dotted box 230, spacer features 228 c and 228 eproperly merge inside a mandrel trench.

The method 100 proceeds to operation 112 by removing the hard mask layer220 through a suitable process, such as an etching process tuned toselectively remove the hard mask layer while the spacer features remain.

Referring to FIGS. 1 and 5 a, the method 100 proceeds to operation 114by forming a material layer 240 on the substrate and within openingsdefined by the spacer features 228 a-e, as illustrated in FIG. 5 a. Inan embodiment, the material layer is deposited over the spacer featuresand is then partially removed such that the top surface of the spacerfeatures is exposed by a procedure, such as chemical mechanicalpolishing (CMP) or etch back. In an embodiment, the material layer usesbottom anti-reflective coating (BARC) or spin-on glass (SOG).

The method 100 proceeds to operation 116 by removing the spacer featuresthrough a suitable process, such as an etching process tuned toselectively remove the spacer features while the material layer 240remains. Wherein the width of the merged trenches is equal to or lessthan twice the thickness of the spacer features, a cut feature is formedafter the spacer features are removed. FIG. 5 b shows the spacerfeatures 228 a-e being removed, leaving a desired pattern on thesubstrate with a cut feature 214.

FIGS. 6 a-11 g show a process flow for a lithographic-spacer processwith cut features according to a second embodiment of the presentdisclosure. In each of FIGS. 6 a-11 g, the figure designated “a” (e.g.,FIG. 6 a) includes a dotted line that defines cross sectional views forthe figures designated “b,” “c,” and so on.

Referring to FIGS. 6 a-6 e, a first layout is formed in the hard masklayer 308 as mandrel trenches. In the present embodiment, a substrateincludes a dielectric layer 302, an inter-layer dielectric (ILD) layer304, and a nitrogen-free anti-reflection coating (NFARC) layer 306. Ahard mask layer 308 is formed on the NFARC layer 306. A first bottommaterial layer 310, a first middle material layer 312, and a firstresist layer 314 are formed for patterning the hard mask layer 308. Inan embodiment, the ILD layer 304 includes an extreme low-k dielectric(ELK) material, the NFARC layer 306 includes a material such as siliconoxide, silicon oxygen carbide, or plasma enhanced chemical vapordeposited silicon oxide, the hard mask layer 308 includes siliconnitride, the bottom material includes a bottom anti-reflection coatingpolymeric material, and the middle material includes silicon containingpolymer.

FIGS. 6 b, 6 c show the device with a patterned resist layer 314according to the first layout as mandrel trenches. FIGS. 6 d, 6 e showthe device after etching through the openings of the patterned resistlayer 314 and thereafter removing the layers 314, 312, and 310, therebyforming a first plurality of trenches in the hard mask layer 308.

Referring to FIGS. 7 a-7 c, a second bottom material layer 320 isdeposited over the hard mask layer 308, a second middle material layer322 is deposited over the second bottom material layer 320, and a secondresist layer 324 is patterned according to a second layout as mandreltrenches. The layers 322, 320, and 308 are to be etched through theopenings of the patterned resist layer 324 to form a second plurality oftrenches in the hard mask layer 308.

FIGS. 8 a-8 c show the merger of the first and second plurality ofmandrel trenches in the hard mask layer 308 after etching the layers322, 320, and 308 through the openings of the patterned resist layer 324in FIGS. 7 b and 7 c, and removing the layers 324, 322, and 320thereafter.

FIGS. 9 a-9 e show spacer features being formed inside and on sidewallsof the merged mandrel trenches. FIGS. 9 b, 9 c show a spacer material330 being deposited over the hard mask layer 308 and on the NFARC layer306. In one example, the spacer material includes titanium nitride.FIGS. 9 d, 9 e show the spacer material 330 from the horizontal surfacesaway from the mandrel sidewalls being removed, such as by an anisotropicetch process, thereby forming spacer features on the sidewalls of themandrel trenches.

FIGS. 10 a-10 e show the hard mask layer 308 being removed, as well asportions of the NFARC layer 306 and the ILD layer 304, with the spacerfeatures formed in FIGS. 9 a-9 e being used as a mask. This can be doneby a suitable process, such as an anisotropic etch process.

FIGS. 11 a-11 g show the desired final pattern being formed in the ILDlayer 304 on the dielectric layer 302. FIGS. 11 b, 11 c show the spacerfeatures 330 and NFARC layer 306 being removed, leaving only thepatterned ILD layer 304 on the dielectric layer 302. This can be done byone or two etching processes, selective to the spacer material and theNFARC material. FIGS. 11 d, 11 e show a material 340 deposited over thepatterned ILD layer 304. For example, the material 340 can be copper,tungsten or silicide for forming metal lines. FIGS. 11 f, 11 g show thedeposited material 340 being planarized to form the final device. Thiscan be done with an etching or chemical mechanical polishing (CMP)process.

Accordingly, the present disclosure provides a method of forming atarget pattern or device by performing a first and a second lithographyprocesses to form mandrel trenches in a hard mask layer, and thereafterperforming spacer and etching processes.

Although not intended to be limiting, an advantage of one or moreembodiments of the present disclosure is that the second layout can beused as not only either a main feature or a cut feature of the targetpattern, but also both of them after proper process scheme. That is, thesecond layout can be used as a new mandrel, a merged portion of thefirst layout, or a cut feature for the first layout so as to achieve adesirable uniformity in density for lithographic exposure. The desirableuniformity in pattern density improves lithography process window.Therefore, the present disclosure is lithography friendly for formingsmall cut features. Moreover, the second layout may be used eitherbefore or after the first layout in performing the method 100 to achievesame result. The new process can be referred to as LLSE (lithography,lithography, spacer, etch). This LLSE process has the advantage of theconventional LELE processes, and has the capability of making smallercut features.

FIGS. 12 a, 12 b illustrate one improvement achieved by the presentdisclosure over a LELE process. For simplicity purposes, a dimension ofa feature in direction X is referred to as the width of the feature, anda dimension of a feature in direction Y is referred to as the length ofthe feature.

Referring now to FIG. 12 a, in a LELE process, a mandrel line within aspacer 256 is cut to two mandrel lines 250 and 252 by a cut feature 254.A distance 255 between one end of mandrel line 250 and one end ofmandrel line 252 is referred to as an End-to-End (EtE) feature which islimited by the length of cut feature 254. Since the width of cut feature254 is constrained by the width of the spacer 256, the minimum length ofcut feature 254 is limited by the lithography process.

Referring now to FIG. 12 b, in an embodiment of the present disclosure,a cut feature 264 is formed as a trench in a second lithography processover two mandrel trenches 266 and 268 formed in a first lithographyprocess. A distance 265 between one end of a target feature 262 and oneend of a target feature 260 is referred to as an End-to-End (EtE)feature, which is limited by the length of cut feature 264. The width ofcut feature 264 is constrained within two mandrel trenches 266 and 268.Because the width of mandrel trenches 266 and 268 is substantiallylarger than the width of spacer 256 from FIG. 12 a, the width of cutfeature 264 can be substantially larger than the width of cut feature254 from FIG. 12 a. As a result, the length of cut feature 264 can besubstantially smaller than the length of cut feature 254 for the samelithography process. Hence, a smaller EtE feature is achieved by anembodiment of the present disclosure.

Thus, the present disclosure provides an embodiment of a method offorming a target pattern. The method includes forming a first materiallayer on a substrate; performing a first patterning process using afirst layout to form a first plurality of trenches in the first materiallayer; performing a second patterning process using a second layout toform a second plurality of trenches in the first material layer; formingspacer features on sidewalls of both the first plurality of trenches andthe second plurality of trenches, the spacer features having athickness; removing the first material layer; etching the substrateusing the spacer features as an etch mask; and thereafter removing thespacer features. The target pattern is to be formed with the firstlayout and the second layout.

The present disclosure also provides another embodiment of a method offorming a target pattern on a substrate. The method includes forming afirst material layer on the substrate; performing a first patterningprocess using a first layout to form a first plurality of trenches inthe first material layer; performing a second patterning process using asecond layout to form a second plurality of trenches in the firstmaterial layer; forming spacer features on sidewalls of both the firstplurality of trenches and the second plurality of trenches, the spacerfeatures having a thickness; removing the first material layer; forminga second material layer on the substrate and within openings defined bythe spacer features; and removing the spacer features. The targetpattern is to be formed with the first layout and the second layout.

The present disclosure provides yet another embodiment of a method offorming a target pattern. The method includes depositing a firstmaterial layer on a substrate; performing a first lithography patterningprocess using a first layout to form a first plurality of trenches inthe first material layer; performing a second lithography patterningprocess using a second layout to form a second plurality of trenches inthe first material layer; forming spacer features on sidewalls of boththe first plurality of trenches and the second plurality of trenchesusing a process including deposition and etching, the spacer featureshaving a thickness; removing the first material layer by an etchingprocess; etching the substrate using the spacer features as an etchmask; and thereafter removing the spacer features using one of: anetching process and a polishing process. The target pattern is to beformed with the first layout and the second layout; the first layoutincludes a first subset of the target pattern; the second layoutincludes a second subset of the target pattern and a cut pattern for thefirst subset; and the cut pattern corresponds to a portion of the secondlayout wherein width of the second layout is less than twice thethickness of the spacer features.

The foregoing outlines features of several embodiments so that those ofordinary skill in the art may better understand the aspects of thepresent disclosure. Those of ordinary skill in the art should appreciatethat they may readily use the present disclosure as a basis fordesigning or modifying other processes and structures for carrying outthe same purposes and/or achieving the same advantages of theembodiments introduced herein. Those of ordinary skill in the art shouldalso realize that such equivalent constructions do not depart from thespirit and scope of the present disclosure, and that they may makevarious changes, substitutions, and alterations herein without departingfrom the spirit and scope of the present disclosure.

What is claimed is:
 1. A method, comprising: forming a first materiallayer on a substrate; performing a first patterning process using afirst layout to form a first plurality of trenches in the first materiallayer, the first layout including a first subset of a target pattern;performing a second patterning process using a second layout to form asecond plurality of trenches in the first material layer, the secondlayout including a second subset of the target pattern and a cut patternfor the first subset; forming spacer features on sidewalls of both thefirst plurality of trenches and the second plurality of trenches, thespacer features having a thickness, wherein the cut pattern correspondsto a first trench of the second plurality whose width is less than twicethe thickness of the spacer features; removing the first material layer;forming a second material layer on the substrate and within openingsdefined by the spacer features; and removing the spacer features.
 2. Themethod of claim 1, wherein the forming of the second material layerincludes: forming the second material layer by spin coating; andselectively etching back the second material layer to expose the spacerfeatures.
 3. The method of claim 1, wherein the forming of the secondmaterial layer includes: forming the second material layer bydeposition; and performing a polishing process to the second materiallayer to expose the spacer features.
 4. The method of claim 1, wherein aportion of the first plurality of trenches is merged with a portion ofthe second plurality of trenches to form a continuous trench.
 5. Themethod of claim 4, wherein the forming of the spacer features onsidewalls of both the first plurality of trenches and the secondplurality of trenches includes forming the spacer features in thecontinuous trench.
 6. The method of claim 1, further comprising: etchingthe substrate with the second material layer as an etch mask.
 7. Themethod of claim 1, wherein the performing of the first patterningprocess using the first layout includes: forming a resist layer over thefirst material layer; patterning the resist layer using the firstlayout, thereby resulting in a resist pattern; and transferring theresist pattern to the first material layer.
 8. The method of claim 1,wherein the performing of the second patterning process using the secondlayout includes: forming a third material layer over the first materiallayer, wherein the third material layer and the first material layer usedifferent materials; forming a resist layer over the third materiallayer; patterning the resist layer using the second layout, therebyresulting in a resist pattern; etching both the first and third materiallayers using the resist pattern as an etch mask; removing the resistpattern; and removing the third material layer.
 9. The method of claim1, wherein the forming of the spacer features includes: depositing aspacer material over the first material layer and on the substrate; andapplying an anisotropic etching process to the spacer material.
 10. Themethod of claim 1, wherein the removing of the spacer features includesan etching process selectively tuned to remove the spacer features whilethe second material layer remains.
 11. A method, comprising: forming afirst material layer on a substrate; performing a first patterningprocess using a first layout to form a first plurality of trenches inthe first material layer; performing a second patterning process using asecond layout to form a second plurality of trenches in the firstmaterial layer; forming spacer features on sidewalls of both the firstplurality of trenches and the second plurality of trenches, the spacerfeatures having a thickness; removing the first material layer; forminga second material layer on the substrate and within openings defined bythe spacer features; and removing the spacer features, wherein: thefirst layout includes a first subset of a target pattern; the secondlayout includes a second subset of the target pattern and a cut patternfor the first subset; and the cut pattern corresponds to a portion ofthe second layout wherein a width of the portion of the second layout isless than twice the thickness of the spacer features.
 12. The method ofclaim 11, wherein a trench of the first plurality interfaces with atrench of the second plurality to form a continuous trench.
 13. Themethod of claim 12, wherein the forming of the spacer features includesforming the spacer features in the continuous trench.
 14. The method ofclaim 11, wherein the forming of the spacer features includes:depositing a spacer material over the first material layer and on thesubstrate; and applying an anisotropic etching process to the spacermaterial.
 15. The method of claim 11, wherein: the spacer featuresinclude titanium nitride; the first material layer includes siliconnitride; the substrate includes a nitrogen-free anti-reflection coatinglayer over an extreme low-k dielectric layer; and the nitrogen-freeanti-reflection coating layer includes a material selected from thegroup consisting of silicon oxide, silicon oxygen carbide, and plasmaenhanced chemical vapor deposited silicon oxide.
 16. The method of claim11, wherein the forming of the second material layer includes:depositing a second material over the substrate and over the spacerfeatures; and performing a polishing process to the second material toexpose the spacer features.
 17. A method, comprising: forming a firstmaterial layer on a substrate; performing a first patterning processusing a first layout to form a first plurality of trenches in the firstmaterial layer; performing a second patterning process using a secondlayout to form a second plurality of trenches in the first materiallayer, wherein a first trench of the second plurality merges with twotrenches of the first plurality; forming spacer features on sidewalls ofboth the first plurality of trenches and the second plurality oftrenches, the spacer features having a thickness, wherein a width of thefirst trench is less than twice the thickness of the spacer featuresthereby the spacer features merge inside the first trench; removing thefirst material layer; forming a second material layer on the substrateand within openings defined by the spacer features; and removing thespacer features.
 18. The method of claim 17, wherein the performing ofthe first patterning process using the first layout includes: forming aresist layer over the first material layer; patterning the resist layerusing the first layout, thereby resulting in a resist pattern; andtransferring the resist pattern to the first material layer.
 19. Themethod of claim 17, wherein the performing of the second patterningprocess using the second layout includes: forming a third material layerover the first material layer, wherein the third material layer and thefirst material layer use different materials; forming a resist layerover the third material layer; patterning the resist layer using thesecond layout, thereby resulting in a resist pattern; transferring theresist pattern to both the first and third material layers; removing theresist pattern; and removing the third material layer.
 20. The method ofclaim 17, wherein the forming of the spacer features includes:depositing a spacer material over the first material layer and on thesubstrate; and applying an anisotropic etching process to the spacermaterial.